Mesomorphic or "liquid crystalline" materials have long been known and have been classified into three types; smectic, nematic, and cholesteric. The cholesteric mesomorphic phase exhibits a number of optical properties which have attracted interest. One property is the scattering of white light, which property varies with temperature over a certain range, depending upon the particular cholesteric material.
This variable scattering of light is manifested as an apparent change in color of the cholesteric material and has been exploited for measuring transient changes in temperature occasioned by infrared radiation, microwave radiation, conduction, and convection. In this use the cholesteric material responds relatively rapidly so that a change in temperature quickly causes a change in the apparent color of the material.
Many attempts have been made to exploit these properties of liquid crystals and a large number of them have been successful.
One area which has attracted considerable attention is the possiblity of constructing thermometric elements such as clinical thermometers which record changes in temperature by visual changes in a plurality of liquid crystals systems, each of which responds to a different temperature, disposed in an orderly fashion on a suitable substrate. Despite major efforts, however, no suitable products have yet been developed.
One of the principal difficulties which has retarded the development of useful thermometric elements has been the necessity of forming a number of different liquid crystal systems each one of which changes in color at a different temperature. Research has advanced to the point where, with some difficulties, systems can be composed which will change color with great accuracy at a fixed temperature, for example at one-half degree intervals over the range from 96.5.degree.F to 104.5.degree.F -- a suitable range for a clinical thermometer. To construct a thermometer capable of recording one-half degree increments in temperature over this range, it has heretofore been necessary to prepare seventeen different systems, each of which varies in the percentage composition of each component. This has caused great difficulties in compounding the compositions and in the cost of production. Additionally, the large number of different compositions involved has introduced significant changes of error.
Another problem has been stability of the products. Many devices which have been proposed are capable of accurately recording temperature changes when first prepared, but lose the ability after standing for an appreciable period of time.
No system has yet been devised which makes it possible to construct thermometric elements capable of recording numerous increments in temperature from one, single, basic composition in a facile and economic manner, accurately and without loss of stability. All systems which have heretofore been disclosed have required several changes in the ingredients or the percent composition of ingredients in the basic system in order to achieve the desired result.